The 3rd Generation Partnership Project (3GPP) is responsible for the standardization of the Universal Mobile Telecommunication System (UMTS) and its Long Term Evolution (LTE). The 3GPP work on LTE is also referred to as Evolved Universal Terrestrial Access Network (E-UTRAN). LTE is a radio access technology for realizing high-speed packet-based communication that can reach high data rates both in the downlink and in the uplink, and is thought of as a next generation mobile communication system relative to UMTS.
LTE uses OFDM (Orthogonal Frequency Division Multiplexing) in the downlink and Discrete Fourier Transform-spread (DFT-spread) OFDM in the uplink. The basic LTE downlink physical resource can thus be seen as a time-frequency grid as illustrated in FIG. 1, where each resource element, i.e. each square in the grid, corresponds to one OFDM subcarrier during one OFDM symbol interval.
In the time domain, LTE downlink transmissions are organized into radio frames of 10 ms, each radio frame consisting of ten equally-sized subframes of length Tsubframe=1 ms. The LTE time-domain structure is illustrated in FIG. 2. Furthermore, resource allocation in LTE is typically described in terms of resource blocks, where a resource block corresponds to one slot (0.5 ms) in the time domain and 12 contiguous subcarriers in the frequency domain. Resource blocks are numbered in the frequency domain, starting with 0 from one end of the system bandwidth.
Downlink transmissions are dynamically scheduled, i.e., in each subframe the base station transmits control information indicating to which terminals data is being transmitted, and upon which resource blocks the data is transmitted, in the current downlink subframe. This control signaling is typically transmitted in a control region comprising the first 1, 2, 3 or 4 OFDM symbols in each subframe. A downlink subframe with 3 OFDM symbols as control region is illustrated in FIG. 3.
Hybrid-ARQ (Automatic Repeat Request), also known as HARQ, is a method used in LTE for correcting transmission errors where data units that are not acknowledged by the receiver are automatically retransmitted. Forward error correction bits are also added to the data to enable the receiver to correct and/or detect if a packet has been incorrectly received. Thus, after receiving downlink data in a subframe, the terminal attempts to decode it and reports to the base station whether the decoding was successful (acknowledgement, ACK) or not (negative acknowledgement, NAK). In case of an unsuccessful decoding attempt, the base station may retransmit the erroneous data.
Uplink control signaling from the terminal to the base station comprises                hybrid-ARQ acknowledgements for received downlink data;        terminal reports related to the downlink channel conditions, used as assistance for the downlink scheduling;        scheduling requests, indicating that a mobile terminal needs uplink resources for uplink data transmissions.        
If the mobile terminal has not been assigned an uplink resource for data transmission, the L1/L2 control information, e.g. channel-status reports, hybrid-ARQ acknowledgments, and scheduling requests, is transmitted in uplink resources, i.e. resource blocks, specifically assigned for uplink L1/L2 control on the Physical Uplink Control CHannel (PUCCH). As illustrated in FIG. 4, these resources are located at the edges of the total available cell bandwidth. Each such resource consists of 12 subcarriers, i.e. one resource block in frequency, within each of the two slots of an uplink subframe. In order to provide frequency diversity, these frequency resources are frequency hopping on the slot boundary, i.e. one resource consists of 12 subcarriers at the upper part of the spectrum within the first slot of a subframe and an equally sized resource at the lower part of the spectrum during the second slot of the subframe or vice versa. If more resources are needed for the uplink L1/L2 control signaling, e.g. in case of very large overall transmission bandwidth supporting a large number of users, additional resources blocks can be assigned next to the previously assigned resource blocks.
The reasons for locating the PUCCH resources at the edges of the overall available spectrum are two-fold:                Together with the frequency hopping described above, this maximizes the frequency diversity experienced by the control signaling.        Assigning uplink resources for the PUCCH at other positions within the spectrum, i.e. not at the edges, would have fragmented the uplink spectrum, making it difficult to assign very wide transmission bandwidths to single mobile terminal and still retain the single-carrier property of the uplink transmission.        
The bandwidth of one resource block during one subframe is too large for the control signaling needs of a single terminal. Therefore, to efficiently exploit the resources set aside for control signaling, multiple terminals can share the same resource block. This is done by assigning the different terminals different orthogonal phase rotations of a cell-specific length-12 frequency-domain sequence and/or different orthogonal time-domain covers covering the subframes within a slot or subframe.
The LTE Rel-8 standard has recently been standardized, supporting bandwidths up to 20 MHz. However, in order to meet the upcoming IMT-Advanced requirements, 3GPP has initiated work on Release 10, also referred to as LTE-Advanced. One of the aims of LTE-Advanced is to support bandwidths larger than 20 MHz. One important requirement on LTE-Advanced is to assure backward compatibility with LTE Rel-8. This should also include spectrum compatibility. That would imply that an LTE-Advanced carrier, wider than 20 MHz, should appear as a number of LTE carriers to an LTE Rel-8 terminal. Each such carrier may be referred to as a component carrier (CC). In particular for early LTE-Advanced deployments it can be expected that there will be a smaller number of LTE-Advanced-capable terminals compared to many LTE legacy terminals. Therefore, it is necessary to assure an efficient use of a wide carrier also for legacy terminals, i.e. that it is possible to implement carriers where legacy terminals can be scheduled in all parts of the wideband LTE-Advanced carrier. The straightforward way to obtain this would be by means of carrier aggregation. Carrier aggregation implies that an LTE-Advanced terminal can receive multiple CCs, where the CCs have, or at least the possibility to have, the same structure as a Rel-8 carrier. Carrier aggregation is illustrated in FIG. 5.
The number of aggregated CCs as well as the bandwidth of the individual CC may be different for uplink and downlink. A symmetric configuration refers to the case where the number of CC in downlink and uplink is the same, whereas an asymmetric configuration refers to the case that the number of CC is different. It is important to note that the number of CC configured in a cell may be different from the number of CC seen by a terminal: A terminal may for example support more downlink CC than uplink CC, even though the cell is configured with the same number of uplink and downlink CC.
In current LTE carrier aggregation terminology, the concepts of “primary serving cell” and “secondary serving cell” (SCell) are also used. A primary serving cell, or PCell, is configured on a primary component carrier, PCC, and a secondary serving cell, or SCell, is configured on a secondary component carrier, SCC. In this context, “component carrier” or “carrier” refers to the physical frequency resource that the cell is configured to use. Thus, whenever this disclosure refers to “a mobile terminal receiving information on a component carrier”, “a base station transmitting on a component carrier” etc, it should be understood this does not preclude a situation where the mobile terminal or base station in question is configured with a primary serving cell and optionally one or more secondary serving cells, and where each serving cell in turn is configured on a component carrier.
Scheduling of the CC is done on the Physical Downlink Control Channel (PDCCH) via downlink assignments. Control information on the PDCCH is formatted as a Downlink Control Information (DCI) message. DCI messages for downlink assignments contain i.a. resource block assignment, modulation and coding scheme related parameters, hybrid-ARQ redundancy version, etc. In addition to those parameters that relate to the actual downlink transmission, most DCI formats for downlink assignments also contain a bit field for Transmit Power Control (TPC) commands. These TPC commands are used to control the uplink power on the corresponding PUCCH that is used to transmit the hybrid-ARQ feedback.
From a UE perspective, both symmetric and asymmetric uplink/downlink CC configurations are supported. For some of the configurations, one may consider the possibility to transmit the uplink control information on multiple PUCCH, PUCCH or multiple uplink CCs. However, this option is likely to result in higher UE power consumption and a dependency on specific UE capabilities. It may also create implementation issues due to inter-modulation products, and would lead to generally higher complexity for implementation and testing. Hence, it is advantageous if the transmission of PUCCH does not depend on the uplink/downlink CC configuration. Therefore it has been agreed for LTE Release 10 to use the design principle that all uplink control information for a UE should be semi-statically mapped onto one specific uplink CC, a so-called “anchor carrier”, or uplink primary component carrier, PCC.
In case the ACK/NACK feedback would be transmitted on PUSCH, it would be beneficial if the ACK/NACK feedback was only transmitted on one CC, for similar reasons as described for PUCCH above.
For the base station to fully utilize all the DL HARQ processes in the UE, it is beneficial to have the individual HARQ bits fed back per HARQ process. In case of carrier aggregation and FDD this means that there will be a maximum of two HARQ processes that need to be fed back per component carrier. In case of TDD, there is also a time component associated with HARQ feedback, so there may be more than two HARQ processes that need to be fed back per CC.
One limiting factor for the downlink transmission is the possibility for the terminal to feedback all the HARQ states to all HARQ process reliably to the base station. In such a situation it is beneficial for the terminal to bundle together the HARQ states for several different HARQ process to generate common HARQ states, i.e. ACK/NACK bundling. This could be done across CCs, across layers (spatial bundling) or over time (temporal bundling). The ACK/NACK bundling may also be done according to any of these examples combined in different ways.
A basic problem with ACK/NACK bundling is that a terminal may miss a DL assignment, which may not be indicated in the bundled response. For instance, assume that the terminal was scheduled on two CCs. On CC 1 the terminal misses the scheduling assignment and will not be aware that it was scheduled, while in the second CC it successfully receives the data. The terminal will, as a result, transmit an ACK, which the base station will assume holds for both CCs, including data in the CC the terminal was not aware of. As a result, data will be lost. The lost data needs to be handled by higher-layer protocols, which typically takes a longer time than hybrid-ARQ retransmissions and is less efficient.
For this reason, it is beneficial to add a DL assignment index (DAI) in the DL assignment, which represents the number of assigned DL CCs. The terminal may, when it has received multiple DL assignments, count the number and compare it with the signalled number in the DAI to see whether it has missed any DL assignments. The terminal may, in the case that it is aware it has missed a DL assignment, provide an indication to the base station, for example by transmitting NACK or using a specific resource.
There is a general need in the art to reduce signaling overhead when transmitting control information.